The neuronal circuitry of the cerebellum appears to be relatively simple and well defined, but the computation performed by this circuitry is not understood. In addition, forms of synaptic plasticity have been defined in the cerebellum that may be responsible for motor learning, but critical questions regarding this plasticity are still unanswered. The objective of the proposed research is to address fundamental issues cerebella circuitry and plasticity through a study of the unique cerebellum of mormyrid electric fish. Two features of the mormyrid cerebellum make it well suited to address these issues: 1) Purkinje cells and cerebellar output cells (cells of the cerebellar nuclei in mammals) are adjacent to each other in mormyrid fish, making possible experiments on integration and plasticity at the important synapse between these two cell types that can not be done in mammals. 2) The regions of termination for climbing fiber and parallel fiber inputs on Purkinje cells are distinct and well separated in the mormyrid but not in the mammal. This separation facilitates the determination of intrinsic differences between the two dendritic regions and the determination of how the two inputs interact to generate synaptic plasticity. The project has two Specific Aims: 1) to determine the basic synaptic and intrinsic physiology of Purkinje and cerebellar output cells in mormyrid electric fish; and 2) to determine the types of plasticity at parallel fiber and Purkinje cell synapses in mormyrid electric fish. The primary method will be whole-cell patch recording in vitro slices, combined with labeling of recorded elements for morphological identification.